1. Field of the Invention
The present invention relates to an improved process for preparing resinous molding materials (more particularly, vinyl aromatic resin, e.g., styrene-based resinous materials) to render them brittle and disintegratable within a short period of time through the action of ultraviolet rays, e.g., sunlight, and to the resinous molding materials thus produced. Said materials are of particular advantage for use in packaging.
2. Description of the Prior Art
With the popularization of self-service sales systems and supermarkets, the demand for so-called one-way or disposable resinous materials for packaging is constantly increasing. Most preferable among the many resinous packaging materials are the polystyrenes because of their excellent properties as a packaging material, for example, good processability, good strength and good sanitary properties for use with food. in fact, they are used as packaging materials in varied ways, for example, as food packaging sheet materials, package cushioning materials,, and package partitioning materials.
These resinous packaging materials, however, are in general very stable and will not undergo substantial degradation over a long period of time, and this has now given rise to the problem of waste-disposal as discussed in more detail hereinbelow. In general, products formed from the above-described packaging materials are bulky and difficult to be rendered highly dense by compression, thus making it difficult to collect and dispose of the same after use. In addition, objects formed from such packaging resinous materials, e.g., cans, boxes, wrappers, cups, forks, spoons, etc., which might be left or thrown away at resort areas or by the sides of highways, are difficult to be decomposed under natural conditions and thus will remain dispersed and unsightly over a long period of time, which not only detracts from beautiful scenery, but also leads to undesired results from environmental and sanitation points of view. Although these wastes are in part recovered at the expense of considerable labor and cost, the amounts have tended to increase through the years.
Heretofore, in attempts to solve the above described problem, polystyrenes have been explored that, after use as packaging containers, are capable of being rendered brittle and disintegratable under the irradiation of sunlight. All such attempts, however, have been directed to achieving this purpose by adding to the polystyrenes various photosensitizers, many of which have toxicity. Accordingly, these prior art methods are accompanied by toxicity problems which are particularly limiting when the photosensitized polystyrenes are to be used as packaging materials for food where strict toxicity and sanitary requirements are in effect.
On the other hand, it has not been practiced in the past to manufacture a hard resin by copolymerizing a vinyl aromatic monomer with 10% by weight or less, with reference to said vinyl aromatic monomer, of a conjugated diene monomer, because such a hard resin has been of no practical value, or rather, nothing has been known about the practical value thereof. In our copending application Ser. No. 291,294 we have described resinous packaging materials obtained by copolymerizing a vinyl aromatic monomer with less than 10% by weight of a conjugated diene monomer based on the total weight of said monomers and several methods of conducting the copolymerization are disclosed. It has now been found, however, that optimum characteristics of high resistance to thermal discoloration (required for molding processes), excellent disintegratability due to decomposition by ultraviolet light and substantial freedom from fisheyes when molded into product can be obtained only in accordance with the present invention.
The resins in accordance with the present invention exhibit, when formed into packaging articles, substantially the same degree of physical properties as do packaging articles made from known polystyrene packaging materials with respect to appearance (including clarity), mechanical properties such as tensile strength and fabrication properties. In addition, when they are used as packaging for products and then discarded, for example on roads, as waste, they self-destruct and are rendered brittle and disintegratable by the action of ultraviolet rays or sunlight and thus it is easy to highly compact and densify the same by compression, thereby facilitating their accumulation and disposition. Moreover, when left outdoors and not recovered, they will be discolored to assume a tan or earth color and become brittle and disintegratable within a very short period of time, breaking into small pieces and into powder which then may be mixed unobtrusively into the soil.